In order to receive and demodulate a signal stably in spite of fluctuations in level of the received signal in an OFDM receiving apparatus that receives an OFDM signal, it is necessary that the amplitude of an input signal be properly controlled. In general, the amplitude of an input signal is controlled by an AGC (Automatic Gain Control) circuit that controls the amplification degree of an amplification circuit contained in a tuner provided in a receiving apparatus.
FIG. 7 is a block diagram showing a conventional OFDM receiving apparatus 100.
As shown in FIG. 7, in the OFDM receiving apparatus 101, a signal received by an antenna 102 is tuned to a desired frequency and amplified by a tuner 103. The received signal thus amplified is converted into a digital signal by an AD converter (shown in FIG. 7 as ADC: Analog to Digital Converter) 104. The digital signal is subjected to a Fourier transform process by a fast Fourier transform section (shown in FIG. 7 as FFT: Fast Fourier Transform) 105, and then subjected to a waveform equalization process by an equalizer 106. Moreover, a demapping section 107 performs a demapping process in accordance with the equalized data supplied from the equalizer 106. Furthermore, a deinterleave/forward error correction section (shown in FIG. 7 as DIL/FEC (Deinterleave/Forward Error Correction)) 108 performs a deinterleaving process and a forward error correction process, so that the received signal is demodulated into unmodulated data.
Further, the digital signal is received by an AGC section 109, and the AGC section 109 determines, in accordance with the received digital signal, such a control voltage (AGC voltage) that the input amplitude is kept at a predetermined level. The amplification degree of a variable gain amplifier contained in the tuner 103 is controlled in accordance with the control voltage.
As compared with a transmission method by which a signal sequence is carried by a single carrier wave (i.e., a single-carrier modulation method), a transmission method, such as an OFDM method, by which a signal sequence is carried by a plurality of carrier waves, has such characteristics as to exhibit much larger fluctuations in amplitude of a time-axis signal. This is because random information data on the frequency axis is subjected to inverse fast Fourier transform (IFFT) by the OFDM method.
In consideration of the above characteristics of the OFDM method, it is necessary for optimum reception conditions that the amplitude of an input signal be so controlled as not to be saturated in the AD converter 104 and as to be at a certain level of reference power superior in such qualities of a received signal as a CN (Carrier to Noise) ratio. The AGC section 109 controls the aforementioned control voltage in the tuner 103, and keeps the amplitude of the input signal at the level of the reference power. In the AGC section 109, the reference power determined at the time of designing is stored. The AGC section 109 determines the control voltage in accordance with an output obtained by making a comparison between the reference power and the input signal.
Meanwhile, since the reference power is set to be optimum in the absence of distortions or spurious components, such qualities of a received signal as a CN ratio and a bit error rate (BER) may not be optimum. A normal OFDM receiving apparatus includes means for removing spurious components and the like with use of a notch filter provided in a demodulator. However, because of the incapability of tuning to the center frequency of the notch, unexpected spurious components cannot be removed. When such spurious components having nonspecific frequencies are generated, the AGC section 109 matches power of the entire band to the reference power. As a result, in the case of a high level of spurious components, the level of a carrier wave having a frequency at which there exist no spurious components is reduced. This causes deterioration in such qualities of a received signal as a CN ratio as the level of carrier waves having other frequencies without spurious components is lowered.
In such a case as described above, such qualities of a received signal as a CN ratio may be improved by so setting the control voltage of the variable gain amplifier of the tuner 103 that the output level is higher than the reference power. In this case, although the signal amplitude may be saturated, the influence of such saturation is traded off for deterioration in level of a desired carrier waves caused by spurious components. Therefore, the performance may be improved.
Further, for such reasons that AGC designing is not perfect, that the amplification degree varies among individual devices, and that the amplification degree varies depending on use conditions such as temperature, such qualities of a received signal as a CN ratio may be improved by performing such an AGC process that the amplitude of an input signal is lower than the reference power. This indicates the possibility of improving performance by changing the reference power in accordance with such qualities of a received signal as a CN ratio and a BER.
Japanese Unexamined Patent Application Publication No. 74721/2006 (Tokukai 2006-74721; published on Mar. 16, 2006) (Document 1) discloses an OFDM demodulation circuit in which all AGC processes are determined based solely on a CN ratio. More specifically, the demodulation circuit monitors a change in CN ratio. Then, in accordance with the change, the demodulation circuit controls the control voltage of a variable gain amplifier of a tuner so that the CN ratio takes on an optimum value.
Japanese Unexamined Patent Application Publication No. 374133/2002 (Tokukai 2002-374133; published on Dec. 26, 2002) (Document 2) discloses an automatic gain control circuit which contains a circuit that monitors the HER after the end of normal AGC control of making a comparison between the power of an input signal and the reference power, and that changes the reference power so that the BER is minimized.
However, according to the demodulation circuit described in Document 1, AGC is performed by calculating the CN ratio of a signal subjected to an FFT process, so that the response of the AGC deteriorates. Moreover, a CN ratio before equalization is used, and the quality of a received signal cannot be derived accurately before equalization. As a result, the signal quality cannot be improved.
Further, according to the automatic gain control circuit described in Document 2, a BER is used as the quality of a received signal instead of a CN ratio. The BER varies in response to changes in input level; therefore, it takes time to measure the BER. As a result, at worst, a burst error occurs at a level-sweeping stage of calculating an optimum level, so that reception is interrupted. Moreover, Document 2 is completely silent about means for minimizing the BER.
According to Documents 1 and 2, in order to improve the CN ratio or the BER, an AGC operation for keeping the amplitude of an input signal at the level of reference power is constantly performed. However, in digital communication using the OFDM or the like, reception is possible as long as the quality of a received signal is at a certain level or above. Therefore, in terms of the performance, there is no advantage in performing an unnecessary AGC trial. Moreover, there is a high possibility of producing a side effect (interaction) with another type of control (e.g., control of power of a tuner according to a CN ratio). Furthermore, an unnecessary AGC trial causes an increase in power consumption.
The following explains the side effect described above. Control of power consumption of a tuner is performed depending on the reception conditions. According to this control, the power consumption of the tuner is reduced under good reception conditions so that the performance of the tuner is reduced, whereas the power consumption of the tuner is increased under poor reception conditions so that the performance of the tuner is enhanced. In other words, the performance of the tuner fluctuates depending on the reception conditions. In cases where adjustment (control) of the reference power is performed at the same time under this control, it may become impossible to distinguish whether the performance fluctuates depending on the quality of reception conditions or in response to the adjustment of the reference power. Furthermore, since the two types of control exist, there is a possibility that neither of them converges.
In cases where the performance is not high (i.e., in cases where a certain level of performance is not ensured), the power of the tuner is not controlled, and the setting for the optimum performance is maintained. Therefore, if the reference power is adjusted only in such a case, there occurs no such side effect for the power control of the tuner.
As described above, the arrangements disclosed in Documents 1 and 2 are not preferable, because AGC cannot be properly performed in accordance with the quality of a received signal.